1. Field of the Invention
This invention relates to fuel cell power plants and more particularly to the operation of fuel cell power plants at low power levels.
2. Description of the Prior Art
Typical fuel cell power plants comprise one or more stacks of fuel cells, the cells within a stack being connected electrically in series. They operate using air to provide oxygen to the cathode and hydrogen to provide fuel to the anodes. After passing through the cells the depleted reactant streams are vented from the system on a continuous basis. Depending upon the size of the power plant, a stack of fuel cells may comprise a half dozen cells or less, or as many as several hundred cells. The air and fuel are usually fed to the cells by one or more manifolds per stack. The cell components are designed to operate within a predetermined voltage band. Voltage above a predetermined maximum must be prevented since excessive voltage can damage external equipment and cause excessively fast corrosion of the cell components particularly the cathodes.
Power output is controlled by varying the load impedance on the cell stack terminals. During periods of relatively high power output, such as when the power plant is operating at or near full power, the fuel flow, air flow, and cell temperature are adjusted as required along "normal" operating curves. These normal operating curves provide a large excess of air such that only about 60 or 70% of the oxygen in the air supplied to the cells is consumed by the cells. (The amount of oxygen consumed by the cells divided by the total amount of oxygen in the air supplied to the cells is hereinafter referred to as the oxygen utilization (U) of the cells.) Normal operating curves are designed to maintain the average cell voltage near or at the maximum permissible in order to obtain peak power plant efficiency. As power is reduced a point will eventually be reached when the normal operating curves cannot maintain the voltage below the permitted maximum voltage. In this specification power levels below this point are referred to as "low" power levels. To operate at these low power levels steps must be taken to reduce the voltage. This is done by degrading cell performance. Obviously it is preferable to do this as cost effectively as possible and without causing serious harm to the power plant components.
One known method for reducing the voltage at low power levels is to reduce the fuel partial pressure to the anode. Commonly owned U.S. Pat. No. 3,379,572 to M. A. Gay describes a hydrogen-oxygen fuel cell system which uses this approach. The fuel cell system of Gay is a closed system, with venting of the reactant stream occurring only occasionally to purge the system of undesirable constituents. During normal operation an excess of pure hydrogen is fed to the anode, and the anode exhaust is continuously recirculated through the cell with makeup fuel being added as the hydrogen is used by the cell. The hydrogen also serves the purpose of carrying away the water produced in the cell. This water is removed from the recirculating anode exhaust stream by a pump separator. Gay cannot reduce the voltage (at low power levels) by merely reducing fuel (hydrogen) flow to the cells because, as is stated, the reduced hydrogen flow would be insufficient to remove all the product water from the cell resulting in dilution of the electrolyte. To solve this problem, instead of reducing the hydrogen flow rate Gay adds a quantity of inert gas (nitrogen) to the recirculating hydrogen stream. This quantity of inert gas now recirculates along with the excess hydrogen during low power level operation, reduces the hydrogen partial pressure, and enables the recirculating stream to carry away the water generated in the fuel cell. As with higher power level operation, pure hydrogen is added to the recirculating anode exhaust as required to make up for the hydrogen used by the cell and to maintain the pressure balance across the cell. When the fuel cell is returned to normal power level operation the recycling anode exhaust stream is vented to get rid of the inert gas. Eventually the recycle stream returns to its normal composition which is basically hydrogen.
The method shown in the Gay patent is limited in its usefulness to non-vented oxygen-hydrogen fuel cells which are not economically suited to large scale power generating systems. It is not suitable for use in the air-hydrogen vented fuel cell system hereinabove discussed and which is the subject matter of the present invention. Furthermore, the method taught by Gay for reducing output voltage at low power levels has little effect on preventing cathode corrosion since it works by increasing the anode potential rather than by lowering the cathode potential.